The development and application of molecular sieve catalysts with shape-selective properties has, without doubt, provided an impetus in recent decades to the development of crude oil processing and petrochemistry. This is so particularly since the discovery of silicon-rich zeolites of medium pore size of the pentasil type.
Pentasil aluminosilicate zeolites are important catalysts in the petroleum and chemical industries and have been applied in processes which (1) lower or eliminate lead and benzene in motor gasoline; (2) replace concentrated liquid or carrier-supported mineral acid catalysts, i.e. sulfuric acid, hydrofluoric acid and phosphoric acid, in aromatic alkylation and olefin hydration processes; and (3) limit the content of aromatics and sulfur in diesel fuels.
Structurally, pentasil zeolites are characterized by an intracrystalline system of mutually crossing channels with a diameter of about 5.5 angstroms. The crossing regions have a very weakly pronounced cage character and are frequently the site of the reaction occurrence. In addition to the acid strength of the acidic centers and their concentration, pore shape and size have an important influence on the activity and selectivity of the conversion of materials and material mixtures.
The size of the pore canals permits the entry and exit of linear and once branched aliphatic molecules and of aromatic molecules with a single benzene ring with up to 10 carbon atoms. Molecules of this class are converted chemically within the pore structure and released as product by diffusion processes. The intracrystalline diffusion rate varies considerably between members of this class due to the differences in molecular size and form. In cases where the activated state of the molecule requires more space than can be satisfied by the crossing regions of the pentasil zeolites, such reactions do not proceed or proceed only with very low probability. This selective property in zeolites is known as shape-selectivity.
The behavior of zeolite catalysts is largely determined by fine differences within the aluminosilicate structure. For example, it is known that the aluminum distribution over the cross section of pentasil zeolite crystals synthesized using organic template compounds is different from that of pentasil zeolite crystals obtained from strictly inorganic synthesis batches (see, for example, A. Tissler et al. Stud. Surf. Sci. Catal. Vol. 46, pages 399-408 (1988)). For the former case, aluminum accumulation in the periphery of the crystals is observed; for the latter, the aluminum over the cross section of the crystals predominates. Structural information of the zeolite provided by X-ray crystal diffraction is therefore not sufficient to characterize the catalytic utility of such materials and needs to be supplemented by more subtle methods such as solid state high resolution nuclear magnetic resonance (NMR) spectroscopy. For a review on the applications of solid state NMR in structural characterization of zeolites, see, Engelhardt, G. et al. "High-Resolution Solid State NMR of Silicates and Zeolites," Wiley; Chichester, England, 1987.
Pentasil zeolites in their protonated form catalyze a variety of reaction types which include: (1) dehydration/hydration (ethers and alkenes from alcohols, alcohols from alkenes), (2) carbon-carbon bond linking reactions (oligomerization of alkenes, condensations of oxygen-containing compounds and alkylation of aromatic compounds and isoparaffins); (3) carbon-carbon bond splitting reactions (cracking processes of paraffins and alkenes); (4) aromatization (synthesis of aromatic compounds from paraffins and alkenes); and (5) isomerizations (backbone and double bond isomerizations).
Methods for the synthesis of aluminosilicates are described extensively in the technical and patent literature (see, for example, Jacobs, P. A. et al. (1987) Stud. Surf. Sci. Cat., Vol. 33, pages 113-146). The reported methods for the synthesis of aluminosilicates, however, suffer from a variety of serious disadvantages which preclude their use for industrial scale, non-polluting production. Examples of such disadvantages include: (1) the use of materials which are toxic and inflammable; (2) formation of undesirable secondary phases, e.g. quartz, in the zeolite product; (3) prolonged reaction times; (4) incomplete reactions; and (5) the use of high temperatures to remove organic contaminants, e.g. structure-directing compounds as quaternary ammonium salts, present in the zeolite lattice which damages the lattice structures leading to a reduction in the catalytic properties. In addition, formation of toxic effluents under conventional synthetic hydrothermal conditions necessitates costly pollution control equipment.
For example, U.S. Pat. No. 3,702,886 discloses the synthesis of silicon-rich zeolites of the pentasil family. The disclosed methods for zeolite synthesis requires the presence of organic, structure-directing compounds or templates in the reaction mixture. Tetralkylammonium compounds, e.g. tetrapropylammonium bromide, are generally used for this purpose.
U.S. Pat. No. 4,257,885 discloses a process for preparing zeolites which omits the use of organic templates. The synthetic processes described therein lead to the desired product under prolonged (several days) reaction times which may not reach completion.
Accordingly, there is a substantial need in the field for improved methods for preparing crystalline aluminosilicates awhich avoid at one or more of the deficiencies mentioned above. Furthermore, there is an acute need in the art for synthetic, crystalline aluminosilicates which display enhanced catalytic properties, long-term stability, and higher selectivity over conventional aluminosilicates in petrochemical processes.